This invention relates to a liquid crystal electro-optic device having good electrical characteristics and angle of visibility characteristics with which a uniform display can be obtained over an entire screen.
As a method of widening the angle of visibility of a liquid crystal electro-optic device, a method wherein the direction of an electric field impressed on a liquid crystal is made substantially parallel to the surface of a substrate (hereinafter referred to as the super TFT method) is disclosed for example in Japanese Unexamined Patent Publication No. H.6-160878. In this case, an electric field is induced between a source electrode and a common electrode formed on one substrate, and the liquid crystal molecules are oriented in the direction of this electric field. Also, in Japanese Unexamined Patent Publication No. H.6-214244, the electric field impressed on the liquid crystal is made uniform by making the height of the electrodes the same as the cell thickness.
In this kind of liquid crystal electro-optic device, because switching is carried out with the long axes of the liquid crystal molecules kept parallel with the substrate, there is no change with angle of visibility in the optical characteristics of the liquid crystal. Consequently, there is less light leakage and contrast reduction and the like resulting from angle of visibility than with conventional TN and STN methods.
However, electrodes of the super TFT method conventionally used have been of a trapezoidal or rectangular structure, and the electric fields produced by these electrodes have been noncontinuous at vertices of the trapezoid or rectangle. Consequently, the electric field impressed on the liquid crystal has changed at certain points. That is, the electric field (electric flux density) has changed suddenly at the vertices of the trapezoid or rectangle. Consequently, switching of the liquid crystal by the electric field has not been carried out evenly in the cell, and a phenomenon of the time taken for the electric field to change from OFF to ON or from ON to OFF (these are respectively called the rise time and the fall time) varying within the cell has appeared.
This is a shortcoming which appears particularly markedly in the super TFT method, wherein a horizontal electric field is used to carry out liquid crystal driving.
The above-mentioned electric field noncontinuity will be explained with reference to FIG. 1. Here, for simplicity, the state of lines of electric force around the electrodes when a voltage is impressed across a pair of parallel electrodes (101, 102) each of a rectangular cross-section of height xe2x80x98axe2x80x99 and width c formed with a spacing 2b between the electrodes on an insulating substrate (103) will be described. (For lines of electric form formed by electric changes, please refer to works on electromagnetism, for example xe2x80x98Electromagnetismxe2x80x99, Kazukiyo Nagata, published by Asakura, or xe2x80x98Detailed Electromagnetic Practicexe2x80x99, Goto and Yamazaki, Kyoritsu publishing.) Here, a direction parallel with the substrate and perpendicular to (the height direction of) the electrodes will be made an x-axis and a direction perpendicular to the surface of the substrate will be made a y-axis. An origin will be so defined that the electrode surfaces parallel with the substrate are at y=0.
(1) In the region y less than 0 (xe2x88x92bxe2x89xa6xc3x97xe2x89xa6b), i.e. the region between the electrodes:
Because electric charge can be regarded as being distributed evenly over the electrode surfaces (104, 105), the lines of electric force (106) here are perpendicular to the electrodes (and parallel with the substrate).
(2) In the region y greater than 0, i.e. the region above the electrodes:
Here, for the sake of simplicity, the state of the lines of electric force in the xy plane will be investigated.
Electric charge can be regarded as being distributed evenly over the electrode surfaces (107, 108).
For any point in the region y greater than 0, the distance from the origin will be written r and the angle made by r and the x-axis will be written xcex8. Also, expressing z as a point in a complex plane using x, y and r, xcex8, the following relationship holds:
z=x+iy=rexp (ixcex8)
Here, to simplify the analysis, a value w will be defined as follows:
w=A log z
(A is a constant of proportionality). If the real and imaginary parts of w are written u and v, then:
w=u+iv=A log z
and
u+iv=A log {rexp (ixcex8)}=A log r+iAxcex8
is obtained. Therefore,
xe2x80x83u=A log r, v=Axcex8
Therefore, the set of curves expressed u=constant in the w plane are the set of curves r=constant in the xy plane, i.e. the set of concentric circles about the origin.
This result is illustrated in FIG. 1, from which it can be seen that the electric field distributions of the electrode side surfaces and the electrode top surfaces are different.
Here, as an example, the electric field between electrodes whose cross-sections are rectangular was shown, but the situation is the same between electrodes whose cross-sections are trapezoidal also. This is because since electric fields are formed perpendicular to the electrode surfaces the electric field of the taper parts and the electric field of the parts parallel with the substrate are noncontinuous at the electrode vertices.
This kind of noncontinuity of the electric field at the electrode vertices is a problem which cannot be ignored when making very small pixels. This is because when as a result of the adoption of very small pixels the number of electrodes increases and the interelectrode distance becomes small the noncontinuous electric field distributes at a high density.
As another method of solving the above-mentioned problem, an invention wherein in order to impress an electric field on the liquid crystal evenly in the cell thickness direction the height of the electrodes is made the same as the thickness of the cell has been proposed, in Japanese Unexamined Patent Publication No. H.6-214244. However, in making extremely tall electrodes, the following technological difficulties arise.
Firstly, when the height of an electrode is made as great as the cell thickness, a large difference in the horizontal direction electrode thickness tends to arise between the top and the base of the electrode. In the super TFT method, wherein the liquid crystal is driven with a horizontal electric field, a difference in the electrode thickness constitutes a difference in the interelectrode distance. Consequently, because the electric field strength in the cell thickness direction varies within the same pixel, driving the liquid crystal becomes difficult.
Secondly, when the electrodes are extremely tall, the coverage of layers formed on top of the electrodes is poor and line breakage tends to occur.
Thirdly, in making very small pixels, with extremely tall electrodes it is difficult to make the horizontal direction film thickness thin and obtain a large taper angle.
Consequently, in making very small pixels, to solve the above-mentioned problems, an electrode structure which can be made by a simple method and which also does not produce a noncontinuous electric field has been being sought.
It is therefore an object of the invention to provide a liquid crystal electro-optic device which has an electrode structure such that noncontinuity of the electric field strength around each pixel electrode is minimized and the display characteristics of the device are thereby improved and which can be made by a simple method.
To achieve this object and other objects, the invention provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein at least one of the electrodes has a curved sectional profile.
The invention also provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein at least one of the electrodes has a semi-circular or semi-elliptical sectional profile.
The invention also provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates including an electrode for liquid crystal driving and a common electrode having parts formed in parallel on the same substrate, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein at least one of the electrodes has a curved sectional profile.
The invention also provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates including an electrode for liquid crystal driving and a common electrode having parts formed in parallel on the same substrate, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein at least one of the electrodes has a semi-circular or semi-elliptical sectional profile.
The invention also provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates including an electrode for liquid crystal driving and a common electrode having parts formed in parallel on the same substrate, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein a nonlinear device is connected to at least one of the electrodes and at least one of the electrodes has a curved sectional profile.
The invention also provides a liquid crystal device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates including an electrode for liquid crystal driving and a common electrode having parts formed in parallel on the same substrate, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein a nonlinear device is connected to at least one of the electrodes and a peripheral driving circuit for driving a liquid crystal material is formed on at least one of the substrates and at least one of the electrodes has a curved sectional profile.
The invention also provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates including an electrode for liquid crystal driving and a common electrode having parts formed in parallel on the same substrate, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein a nonlinear device is connected to at least one of the electrodes and at least one of the electrodes has a semi-circular or semi-elliptical sectional profile.
The invention also provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates including an electrode for liquid crystal driving and a common electrode having parts formed in parallel on the same substrate, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein a nonlinear device is connected to at least one of the electrodes and a peripheral driving circuit for driving a liquid crystal material is formed on at least one of the substrates and at least one of the electrodes has a semicircular or semi-elliptical sectional profile.
The invention also provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein at least one of the electrodes has a curved sectional profile and the tangential direction of a line of electric force around the surface of this electrode changes continuously over the entire surface of the electrode.
The invention also provides a liquid crystal electro-optic device comprising a pair of substrates of which at least one is transparent, electrodes formed on at least one of the substrates, a liquid crystal layer held between the substrates, and electric field impressing means for impressing an electric field on the liquid crystal layer by way of the electrodes, wherein at least one of the electrodes has a semi-circular or semi-elliptical sectional profile and the tangential direction of a line of electric force around the surface of this electrode changes continuously over the entire surface of the electrode.
An example of a construction using the invention disclosed in this specification is shown in FIG. 4 and FIG. 5. FIG. 4 is a schematic plan view of a pixel part of an active matrix type liquid crystal electro-optic device wherein nematic liquid crystal is used and this liquid crystal material is driven with a horizontal electric field and a-Si TFTs are used as the driving devices, and FIG. 5 is a sectional view on the line A-Axe2x80x2 in FIG. 4.
In the construction shown in FIG. 4 and FIG. 5, 401 denotes first and second substrates, 402 a base SiO2 film, 403 a gate electrode, 404 a common electrode, 405 a gate insulating film, 406 a-Si, 407 a source electrode, 408 a drain electrode, 409 a protective layer, 411 an orienting film, 412 a polarizing plate and 413 a liquid crystal layer.
The liquid crystal electro-optic device of this invention is one wherein a liquid crystal material is operated by controlling the strength of an electric field (a horizontal electric field) between a drain electrode and a common electrode formed on a TFT substrate.
For the above-mentioned first and second substrates, a transparent material having a certain degree of strength with respect to outside forces, for example an inorganic material such as glass or quartz, is used. For the substrate on which the TFTs are formed (hereinafter called the TFT substrate), non-alkali glass or quartz glass is used. When a lightweight liquid crystal electro-optic device is to be made, a film having little birefringence, for example PES (Poly Ethylene Sulfate) or the like also can be used.
As the method by which the liquid crystal material is driven, the multiplex method or the active matrix method may be used.
With the multiplex method, all that need be formed on the first substrate are electrodes for display and reference electrodes, but in the case of the active matrix method, in addition to these a nonlinear device, for example a thin film transistor (TFT) or a diode, is formed for each pixel as a switching device.
As the TFT, a transistor in which amorphous silicon or polysilicon (polycrystalline silicon) is used as an active layer can be used. In the case of the active matrix method, as the construction of the driving device, a known construction such as the stagger type or the reverse stagger type can be used. In the case of a transistor wherein polysilicon is used, it is possible to form a peripheral driving circuit for driving the liquid crystal material on the substrate on which the TFTs are formed. The peripheral driving circuit can be formed in the same process as that by which the TFTs are made. This peripheral driving circuit is made up of complementary devices wherein n-channel and p-channel transistors are combined.
As the device electrodes, Cr, Al, ITO and Ta can be used. The sectional profiles of the electrodes are made smoothly sloping or curved by a method shown below. A sectional profile forming a smoothly sloping surface or a curved surface shown in this specification can be made by a dry process or a wet process. Examples of dry processes include:
(a) methods wherein anisotropic plasma etching and isotropic plasma etching are combined; and
(b) methods wherein plasma isotropic etching is carried out using a mask.
As a method of category (a) above, a mask is patterned on an electrode and anisotropic plasma etching is carried out. The mask is then removed, and resist is coated onto parts not to be isotropically plasma etched. After that, isotropic plasma etching is carried out without a mask on parts to be given a curved sectional profile. In this way, projecting parts are shaved off and it is possible to make an electrode having a smoothly sloping curved sectional profile. After that, the resist is removed. As a method of category (b) above, it is possible to obtain a neat arcuate sectional profile by suitably setting a discharge gas voltage.
In a wet process, on the other hand, as the resist, one whose etching selection ratio is not much different from that of the electrode being etched is used. Also, a resist whose taper angle is somewhat small is used. When this is done, the mask and the electrode being etched are etched by wet etching at about the same rate. In this way, it is possible to make an electrode having a smoothly sloping curved sectional profile with rounded vertices.
The above-mentioned methods are just examples of methods for making electrodes having smoothly sloping curved sectional profiles, and the method by which an electrode having a smoothly sloping curved sectional profile of the invention is made is not limited to these methods.
If one of the electrode materials mentioned above is used, by forming an oxide film of the metal constituting the electrode material on the electrode surface by a method such as anodic oxidation after the curved sectional profile is formed as described above, it is possible to make this an interlayer insulating film. In this way, it is possible to improve interelectrode insulation even in cases of constructions wherein adjacent electrodes or electrode patterns overlap.
Also, it is possible to use silicon oxide (SiO2) or silicon nitride (SiN) as interlayer insulating films and TFT protecting layers.
For the opposing substrate, the same material as that used for the substrate on which the TFTs are formed can be used. Also, although it is not particularly necessary to form any electrodes on the opposing substrate, in some cases electrodes 414 may be formed on all or part of the opposing substrate as shown in. FIG. 8A. As the electrode material in this case, besides the above-mentioned metals, a material having transparency, for example ITO or the like, can be used.
To improve contrast by blocking light from parts not contributing to display, a black matrix 415 is formed on the opposing substrate or the TFT substrate or both substrates using a metal such as Cr or a resin material in which a black pigment has been dispersed as shown in FIG. 8B. Also, in the case of color display, R (red), G (green), B (blue) or C (cyan), M (magenta), Y (yellow) color filters are formed in positions corresponding to respective pixels. As the arrangement of the colors of the color filters, a stripe arrangement or a delta arrangement or the like can be used.
After that, an orienting process is carried out on the substrate on which the driving devices are formed and on the opposing substrate. This orienting process is carried out so that the liquid crystal molecules are parallel with the substrate and oriented uniaxially. As the orienting process, rubbing treatment wherein the substrate surface or the surface of an organic resin film of nylon or polyimide or the like (orienting film) (411) formed on the substrate is rubbed in one direction is effective.
The rubbing direction differs according to the liquid crystal material (413) used, and on the TFT substrate side, in the case of a liquid crystal material whose dielectric constant anisotropy is positive, the rubbing direction is made a direction not parallel to the electric field, and preferably at 45xc2x0 to the electric field. In the case of a material whose dielectric constant anisotropy is negative, the rubbing direction is made a direction not orthogonal to the electric field, and preferably 45xc2x0 to the electric field. Rubbing of the opposing substrate side is carried out in a direction parallel or oppositely parallel to the rubbing direction of the TFT substrate.
The pair of substrates thus made are brought face-to-face with each other with a fixed spacing therebetween to form a liquid crystal cell. A sealing agent (not shown) as an adhesive is formed in a predetermined pattern on one of the substrates. As the sealing agent, a resin material hardened thermally or by ultraviolet rays is used. As this resin material, an epoxy or urethane acrylate material can be used. Spacers (not shown) for maintaining the spacing between the two substrates over the whole cell are distributed on the other substrate. After the sealing agent is hardened, the liquid crystal material is injected into the liquid crystal cell by vacuum injection or the like.
Examples of liquid crystal materials which can be used in this invention include nematic, cholesteric and smectic materials, but using a nematic material is particularly preferable. Also, from among nematic liquid crystals, one whose dielectric constant anisotropy is positive or one whose dielectric constant anisotropy is negative is suitably chosen according to the driving method. Also, to reduce the influence of birefringence, a liquid crystal material whose refractive index anisotropy is small is preferable.
In a liquid crystal electro-optic device of the invention, to carry out display utilizing the birefringence of the liquid crystal material, a pair of polarizing plates (412) are arranged with their optical axes intersecting orthogonally and the liquid crystal cell is sandwiched between this pair of polarizing plates. At this time, the orientation direction of the liquid crystal material is parallel with the optical axis of the analyzer, i.e. the polarizing plate nearer the light source.
In a liquid crystal electro-optic device made in this way, the orientation of the liquid crystal material is such that when there is no electric field the long axis of the liquid crystal material is uniaxially oriented in parallel with the substrate and in parallel with the rubbing direction. Then, when an electric field is impressed, the liquid crystal molecules in the vicinities of the orienting film surfaces, which are subject to a strong orientation restricting force, remain parallel with the rubbing direction while the optical axes of the liquid crystal molecules in the vicinity of the middle of the liquid crystal layer, which are only subject to a weak orientation restricting force, are changed by the electric field. When a liquid crystal material whose dielectric constant anisotropy is positive is used, the long axes of the liquid crystal molecules become oriented in parallel with the electric field direction, and when the dielectric constant anisotropy is negative the long axes of the liquid crystal molecules become oriented perpendicular to the electric field direction.
Consequently, with respect to light passing through the liquid crystal electro-optic device, because when there is no electric field the orientation of the liquid crystal material inside the cell is parallel with the optical axis of the analyzer, incident light cannot pass through the polarizer and the amount of light passing through at this time is zero. When an electric field is impressed, on the other hand, the orientation of the optical axis of the liquid crystal material changes and consequently incident light becomes elliptically polarized light and passes through the polarizer.
In the construction described above, two polarizing plates are used, but if a reflecting plate made of metal or the like is formed on one of the two substrates, it is possible to make the liquid crystal electro-optic device using only one polarizing plate, and a bright display can be realized. The metallic reflecting plate can also double as for example a pixel electrode.
When a liquid crystal electro-optic device is constructed according to this invention, compared to electrodes having rectangular or trapezoidal sectional profiles which have been used in conventional liquid crystal electro-optic devices, the electric field around the electrodes is continuous. This continuity of the electric field is clear from the state of lines of electric force around the electrodes when a voltage is impressed on the electrodes. The state of lines of electric force around electrodes will now be described in detail with reference to FIG. 2.
First, for simplicity, a case wherein point charges q1, q2 exist at points O1, O2 will be considered.
Here, the straight line joining O1, O2 will be taken as an x-axis and a direction perpendicular to the x-axis will be made a y-axis. An origin will be defined as the point half-way between the points O1, O2.
A line of electric force passing through any point P as shown in FIG. 2 will be considered. This is in the plane formed by the point P and the points O1, O2.
When this line of electric force is rotated about the O1, O2 axis, a surface of rotation is obtained, and the electric flux passing through any cross-section of this surface of rotation should be constant.
The electric flux passing through a vertical section S passing through P will now be obtained.
If the angles made by the lines O1P and O2P and the O1O2 axis are respectively written xcex81, xcex82, the electric flux xcfx861 passing through S due to q1 is:
xcfx861=q1xc2x72xcfx80(1xe2x88x92cos xcex81)/4xcfx80
and the electric flux xcfx862 passing through S due to q2 is:
xcfx862=q2xc2x72xcfx80(1xe2x88x92cos xcex82)/4xcfx80
and therefore the total electric flux xcfx86 passing through S is given by:
xcfx86=1/2{(q1+q2)xe2x88x92(q1 cos xcex81+q2 cos xcex82)}
Therefore, on one line of electric force,
q1 cos xcex81+q2 cos xcex82=constant
If
q1=xe2x88x92q2,
on the line of electric force there is the relationship:
cos xcex81xe2x88x92 cos xcex82=constant
Lines of electric force (110) and equipotential surfaces (111) formed by this pair of point charges are shown in FIG. 3.
The distribution of these lines of electric force is the same even if charges the same size as the above-mentioned point charges are distributed on a conducting surface of radius xe2x80x98axe2x80x99. Also, in the region yxe2x89xa70, it can be approximated to an electric field created by two semi-circular electrodes. Therefore, if the electrode sectional profiles are semi-circular, the distribution of the lines of electric force is continuous with respect to the cell thickness direction.
In the above description, an example wherein the entire sectional profile of the electrode has a circular curvature was shown, but the invention is not limited to this and the same effects can be obtained with an electrode having an elliptical curvature. Also, the sectional profile does not have to form a regular semicircle, and the same effects can be obtained with a sectional profile forming an arc. The electrode edge section may have a curved surface of a circular arc shape or the like. An electrode having a sectional profile having a polygonal shape with gentle boundary changes may of course also be used.
Also, films formed on thin films such as electrodes having smoothly sloping curved sectioned profiles have good coverage, because of roundness of the thin films. Therefore, there is also the effect of preventing mixing in of impurities and line breakage caused by poor coverage.
The technique of this invention of making the sectional profile of an electrode curved or smoothly sloping can of course be applied not only to the above-mentioned a-Si type TFTs but also to poly-Si type TFTs.
In particular, when poly-Si is used for the active layer of a TFT, because the carrier mobility of the active layer is larger than when a-Si is used for the active layer and consequently the same characteristics as an a-Si transistor can be obtained with a smaller device region, devices can be made small and therefore a high percentage aperture can be realized. Also, in impressing a horizontal electric field, a higher response speed can be realized when poly-Si, having a large carrier mobility, is used for the TFT active layer. Furthermore, when poly-Si is used, it is possible to also form a peripheral driving circuit for driving the liquid crystal material on the substrate and this contributes to reduction of the number of steps required to manufacture the device, improvement of yield and reduction of the price of the device.
The invention has been discussed above with reference to a liquid crystal electro-optic device of a type wherein a horizontal electric field is impressed on a liquid crystal material; however, the invention is not limited to this and can also be used in a liquid crystal electro-optic device of a type wherein a vertical electric field is impressed on liquid crystal material, for example a conventional TN type or the like, whereby disturbances in the electric field at the ends can be reduced and it is possible to make an electro-optic device having good coverage.